1. Field of the Invention
The invention relates to a power amplifier stage of the voltage follower type which receives an input voltage signal and supplies an output signal, which stage comprises at the output a first MOS power transistor whose source is coupled to a common mode line in series with a second MOS power transistor whose drain is coupled to a positive supply line, which two transistors are of the same conductivity type and whose common junction point supplies said output signal of the stage, which stage further comprises a first amplifier supplying a first control signal applied to the gate of the first MOS power transistor, and a second amplifier supplying a second control signal applied to the gate of the second MOS power transistor, which first and second amplifier each receive the input signal at a respective first input, with which signal the output signal of the stage, applied to a respective second input, is compared.
2. Description of the Related Art
A power amplifier stage with a voltage gain equal to 1 is known from the document EP-A-0 492 374, whose envisaged application is the reproduction of acoustic signals.
MOS power transistors are also quite suitable for forming the output of the power amplifier stage in an application specifically designed for motor control, because of their robustness in the presence of an inductive charge. However, this type of transistor has a high gate capacitance, currently of the order of about a hundred pF, so that the control of these transistors renders it necessary to use an amplifier capable of charging and discharging this gate capacitance in a time span compatible with the desired specifications.
It is equally desirable for this same application that the conductive state of the MOS output transistors should be controlled in a very emphatic manner, i.e. the resistance to current passage should be as low as possible, for example Ron below 1 Ohm.
Under these circumstances, a rigorous time management of the conductive and blocked states of these two power transistors must be ensured so as to avoid a risk of destruction of these transistors, which would be the case if the two power transistors were conductive simultaneously even for a short moment.
Finally, a power amplifier stage of the voltage follower type should also comply with a current consumption requirement, which must be as low as possible when the stage is idle, i.e. when the input voltage is equal to the output voltage of the stage. By contrast, major currents must be possible at the output for transitory states during which the amplifier is out of balance, in response to input/output imbalance voltages as small as 100 or 200 mV, and with a response time which should comply with the requirements of the application.
The known amplifier stage complies only partly with these requirements since it has been specially designed for avoiding crossover distortion and not for minimizing the idle current consumption. In the known amplifier stage, in fact, the capability of reacting rapidly at the output in response to a variation in the input voltage is directly connected to the value of the idle current. This idle current, accordingly, cannot be chosen to be as low as desired.